EP1413900B1 - Lenses for electronic imaging systems - Google Patents

Description

FIELD OF THE INVENTION
• This invention relates to lenses for use with electronic imaging systems, e.g., systems employing charged coupled devices (CCDs) or similar light sensitive electronic components. Such systems are well known in the art and descriptions thereof can be found in various references, including Rose et al., "Physical Limits to the Performance of Imaging Systems," Physics Today, September 1989, pages 24-32 and the references cited therein; and Sequin et al., "Charge Transfer Devices," Advances in Electronics and Electron Physics, suppl. 8, L. Marton editor, Academic Press, New York, 1975, the relevant portions of all of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
• Electronic imaging systems require lens systems which are capable of producing high quality images which have a small size, i.e., they require lens systems having short focal lengths. For example, CCDs having a diagonal of approximately 5.5 mm (known as 1/3 inch CCDs) are widely available. Within this small size, a typical CCD will have over 200,000 pixels, thus giving the device a resolution on the order of 40 cycles per millimeter at the surface of the CCD.
• Short focal length lens systems typically comprise small lens elements. Such elements, if they are too small, can be difficult to handle and assemble into a finished unit. Cost is always a dominant factor for lenses for electronic imaging systems, especially where the system is to be part of a mass marketed product. Because CCDs have a high level of resolution, lenses used with such devices must be of high optical quality. This requirement exacerbates the cost problem. In particular, the requirement puts a high premium on achieving a high level of optical performance with a minimum of lens elements.
• WO96/17265 discloses a lens system corresponding to the preamble of claim 1
SUMMARY OF THE INVENTION
• The lens system according to the present invention is defined by the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
• Figures 1-4 are schematic side views of lens systems constructed in accordance with the invention.
• These drawings, which are incorporated in and constitute part of the specification, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the invention. It is to be understood, of course, that both the drawings and the description are explanatory only and are not restrictive of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
• As discussed above, the lens systems of the present invention consist of two lens elements.
• The first lens element has a negative optical power, i.e., f₁ < 0, and preferably has the following properties: $$\left|{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">f</figure-callout>}}_1\right|/{\mathrm{f}}_0>1.0;$$

  

  $${\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">t</figure-callout>}}_1/{\mathrm{f}}_0>0.5;$$

  

  
  and $${\mathrm{<figure-callout\; id="1"\; label="D"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">D</figure-callout>}}_1/{\mathrm{D}}_{\mathrm{EP}}>2.5;$$

  

  
  where f₀ is the focal length of the lens system, f₁ is the focal length of the first lens element, t₁ is the thickness of the first lens element, D₁ is the diameter of the first lens element, and D_EP is the diameter of the entrance pupil of the lens system. The use of a thick front element provides correction for the field curvature of the system.
• In certain embodiments, the lens systems of the invention satisfy the following relationships: $$\left|{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">f</figure-callout>}}_1\right|/{\mathrm{f}}_0>1.5;$$

  

  
  and $${\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">t</figure-callout>}}_1/{\mathrm{f}}_0>\mathrm{0.7.}$$

  

  
  In connection with these embodiments, the t₁/f₀ ratio is most preferably greater than 1.0.
• As used herein and in the claims, the diameter of a lens element is the element's largest clear aperture and the diameter of the entrance pupil of a lens system is the system's equivalent singlet focal length divided by the system's infinity f-number. Based on these definitions, the lens systems of Examples 1-4 set forth below have D₁ values of 9.5, 9.6, 11.9, and 5.6 mm, and D_EP values of 1.5, 1.5, 2.3, and 1.5 mm, so that their D₁/D_EP ratios are 6.3, 6.4, 5.2, and 3.7, respectively. Preferably, the D₁/D_EP ratio is greater than 3.0.
• The second lens element has a positive optical power, i.e., f₂ > 0, and preferably has the following properties: $${\mathrm{f}}_2/{\mathrm{f}}_0<2.0;$$

  

  $${\mathrm{d}}_{12}/{\mathrm{f}}_0>0.25;$$

  

  $${\mathrm{D}}_2/{\mathrm{D}}_{\mathrm{EP}}>1.3;$$

  

  and $${\mathrm{t}}_2/{\mathrm{f}}_0>0.5;$$

  

  
  where f₂ is the focal length of the second lens element, d₁₂ is the distance between the first and second lens elements, D₂ is the diameter of the second lens element, and t₂ is the thickness of the second lens element. Based on the above definition of the diameter of a lens element, the lens systems of Examples 1-4 set forth below have D₂ values of 4.0, 4.0, 4.5, and 3.9 mm, and D₂/D_EP ratios of 2.7, 2.7, 2.0, and 2.6, respectively. Preferably, the D₂/D_EP ratio is greater than 1.5.
• In certain embodiments, the lens systems of the invention satisfy the following relationships: $${\mathrm{f}}_2/{\mathrm{f}}_0<1.6;$$

  

  $${\mathrm{d}}_{12}/{\mathrm{f}}_0>0.5;$$

  

  
  and $${\mathrm{D}}_2/{\mathrm{D}}_{\mathrm{EP}}>\mathrm{1.5.}$$

  
• In some embodiments, the second lens element is a refractive-diffractive hybrid element. The fabrication of such elements is well known in the art. See, for example, C. Londono, "Design and Fabrication of Surface Relief Diffractive Optical Elements, or Kinoforms, with Examples for Optical Athermalization," Ph.D. diss., Tufts University, 1992, and the references cited therein, the relevant portions of all of which are incorporated herein by reference. Diffractive surfaces have the problem of diffraction efficiency, i.e., all orders do not come to a perfect focus. This effect is often seen as "glare". For an electronic imaging system application, the diffraction efficiency problem can be addressed by digital processing of the electronic image.
• When the second lens element is a refractive-diffractive hybrid, the ratio of f₂/f₀, where f₂ includes the contribution of the diffractive surface, is preferably greater than 1.0, e.g., the ratio is approximately 1.5. When a hybrid is not used, the f₂/f₀ ratio is preferably less than 1.0.
• The use of a refractive-diffractive hybrid element for the second lens element provides color correction for the lens system and allows both the first and second lens elements to be composed of a low dispersion material such as acrylic. If such a hybrid element is not used, the first lens element should have a higher dispersion than the second lens element. For example, the first lens element can be composed of styrene and the second lens element can be composed of acrylic for such embodiments. Other plastics can, of course, be used if desired. For example, in place of styrene, polycarbonates and copolymers of polystyrene and acrylic (e.g., NAS) having flint-like dispersions can be used. See The Handbook of Plastic Optics, U.S. Precision Lens, Inc., Cincinnati, Ohio, 1983, pages 17-29. The highest levels of color correction (both axial and lateral) are generally achieved when the second lens element is a refractive-diffractive hybrid element.
• In certain preferred embodiments, the first lens element has a spherical surface and a conic surface and the second lens element also has a spherical surface and a conic surface. This arrangement facilitates manufacture of the lens system.
• In other embodiments, the second lens element can have two conic surfaces, and the first lens element can have an object side surface which is spherical and an image side surface which in some embodiments is a conic and in other embodiments is a general asphere. Specifically, the image side surface of the first lens element can be a conic when the second lens element is a refractive-diffractive hybrid element. Otherwise, for these embodiments, the image side surface of the first lens element is typically a general asphere to facilitate aberration correction. Conic surfaces are preferred to general aspherical surfaces since the polynomial used to define a general aspherical surface (see below) can lead to undesired surface configurations if the diameter is extended beyond the clear aperture, while a conic surface does not suffer from this problem.
• The terms "spherical", "conic", and "general asphere" are used herein and in the claims in accordance with their conventional meanings in terms of a lens surface equation of the following type: $$z=\frac{c{y}^2}{1+{\left[1-\left(1+k\right)c^2{y}^2\right]}^{1/2}}+D{y}^4+E{y}^6+F{y}^8+G{y}^{10}+H{y}^{12}+I{y}^{14}$$

  

  
  where z is the surface sag at a distance y from the optical axis of the system, c is the curvature of the lens at the optical axis, and k is a conic constant
• Thus, a surface is spherical if "k" and "D" through "I" are all zero, a surface is a conic if "k" is non-zero and "D" through "I" are all zero, and a surface is a general asphere if at least one of "D" through "I" is non-zero. A surface for which "k" is non-zero and at least one of "D" through "I" is non-zero, is a general aspherical surface. Other equations besides that set forth above can of course be used to describe a surface of a lens element, and analogous considerations regarding the values of the parameters of such an equation will apply in determining whether a particular surface is a spherical surface, a conic surface, or a general aspherical surface.
• Figures 1 to 4 illustrate various lens systems constructed in accordance with the invention. Corresponding prescriptions and optical properties appear in Tables 1 to 4, respectively. The Hoya designation is used for the glass plate employed in Figures 2 and 4. Equivalent glasses made by other manufacturers can be used in the practice of the invention. Industry acceptable materials are used for the plastic elements.
• The aspheric coefficients set forth in the Tables 1 and 2 are for use in the equation set forth above. The abbreviations used in the tables are as follows:

EFL

effective focal length

FVD

front vertex distance

f/

f-number

ENP

entrance pupil as seen from the long conjugate

BRL

barrel length

OBJ HT

object height

MAG

magnification

STOP

location and size of aperture stop

IMD

image distance

OBD

object distance

OVL

overall length.

• The designation "c" associated with various of the surfaces in the tables represents a conic surface. The designation "a" associated with surface 2 of Tables 1 and 2 represents a general aspherical surface. Surfaces 6 and 7 in Table 3 represent a diffractive surface. The asterisks used in this table represent the index of refraction and the Abbe numbers used in the Sweatt model for a diffractive surface, e.g., a N_e value of 9999 and a V_e value of -3.4. See W.C. Sweatt, "Mathematical Equivalence between a Holographic Optical Element and an Ultra High Index Lens," Journal of the Optical Society of America, 69:486-487, 1979. Although shown as a separate element in Figure 3, the diffractive surface is in fact part of the second lens element. Surface 3 in Tables 1-4 is a vignetting surface. All dimensions given in the tables are in millimeters.
• As is conventional, the figures are drawn with the long conjugate on the left and the short conjugate on the right. Accordingly, in the typical application of the invention, the object to be viewed will be on the left and an electronic imaging system, e.g., a system employing a CCD, will be on the right.
• Although specific embodiments of the invention have been described and illustrated, it is to be understood that the invention is as defined in the appended claims.

  

  

  

  

Claims (16)

1. A lens system for forming an image of an object, said system consisting in order from its object side to its image side of one of the following combinations, namely:

   (i) a negative first lens element having a spherical surface and a general aspherical surface, and a positive second lens element having two conic surfaces;

   (ii) a negative first lens element having a spherical surface and a conic surface, and a positive second lens element having two conic surfaces;

   (iii) a negative first lens element having a spherical surface and a conic surface, and a positive second lens element having a spherical surface and a conic surface, characterized in that

   
   the system has a focal length f₀, the first lens element has a focal length f₁ and a thickness t₁, and the second lens element has a focal length f₂ and a thickness t₂, said second lens element being spaced from the first lens element by a distance d₁₂,
   wherein:

   (a) $${\mathrm{t}}_1/{\mathrm{f}}_0>0.5;$$

   

   
   and

   (b) $${\mathrm{t}}_2/{\mathrm{f}}_0>0.5;$$

   

   
   and wherein the terms "spherical surface", "conic surface", and "general aspherical surface" are used herein in terms of a lens surface equation of the type: $$z=\frac{c{y}^2}{1+{\left[1-\left(1+k\right)c^2{y}^2\right]}^{1/2}}+D{y}^4+E{y}^6+F{y}^8+G{y}^{10}+H{y}^{12}+I{y}^{14}$$

   

   
   where z is the surface sag at a distance y from the optical axis of the system, c is the curvature of the lens at the optical axis, and k is a conic constant, a surface being spherical if "k" and "D" through "I" are all zero, a surface being a conic if "k" is non-zero and "D" through "I" are all zero, and a surface being a general asphere if at least one of the "D" through "I" is non-zero.
2. The lens system of Claim 1, wherein when the system consists of combination (ii) the second lens element is a refractive diffractive hybrid element, and when the system consists of combination (iii) the conic surface is on the image side of each of the first and second lens elements.
3. The lens system of Claim 1 wherein: $${\mathrm{d}}_{12}/{\mathrm{f}}_0>\mathrm{0.25.}$$

   
4. The lens system of Claim 1 wherein: $$\left|{\mathrm{f}}_1\right|/{\mathrm{f}}_0>\mathrm{1.0.}$$

   
5. The lens system of Claim 1, wherein: $$\left|{\mathrm{f}}_1\right|/{\mathrm{f}}_0>1.5;$$

   

   $${\mathrm{t}}_1/{\mathrm{f}}_0>0.7;$$

   

   
   and $${\mathrm{d}}_{12}/{\mathrm{f}}_0>\mathrm{0.5.}$$

   
6. The lens system of any preceding claim, wherein: $${\mathrm{f}}_2/{\mathrm{f}}_0<2.0$$

   
7. The lens system of Claim 6, wherein: $${\mathrm{f}}_2/{\mathrm{f}}_0<1.6$$

   
8. The lens system of any preceding Claim, wherein the first lens element has a higher dispersion than the second lens element.
9. The lens system of Claim 8, wherein the first lens element is composed of styrene and the second lens element is composed of acrylic.
10. The lens system of any preceding Claim, wherein the second lens element is a refractive-diffractive hybrid element.
11. The lens system of Claim 10, wherein the first and second lens elements are composed of acrylic.
12. The lens system of Claim 10, wherein: $${\mathrm{f}}_2/{\mathrm{f}}_0>\mathrm{1.0.}$$

    
13. The lens system of any preceding Claim, wherein the lens system has an entrance pupil whose diameter is D_EP, the first lens element has a diameter D₁, and the second lens element has a diameter D₂, where: $${\mathrm{D}}_1/{\mathrm{D}}_{\mathrm{EP}}>2.5;$$

    

    
    and $${\mathrm{D}}_2/{\mathrm{D}}_{\mathrm{EP}}>\mathrm{1.3.}$$

    
14. The lens system of Claim 13, wherein: $${\mathrm{D}}_1/{\mathrm{D}}_{\mathrm{EP}}>3.0;$$

    

    
    and $${\mathrm{D}}_2/{\mathrm{D}}_{\mathrm{EP}}>\mathrm{1.5.}$$

    
15. An optical system comprising a lens system for forming an image of an object and an electronic imaging system for detecting said image, said lens system being the lens system of any preceding claim.
16. The optical system of Claim 15, wherein the electronic imaging system is a charged coupled device.

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